The present invention relates, in general, to a sending unit for a float operated liquid level sensor employing a thick-film, printed circuit resistor card, and, more specifically, to a fuel level sender assembly for a fuel tank constructed in a manner to eliminate wearing out of the resistor card by vibrations of the float.
Fuel level sensing in automobile fuel tanks is typically accomplished using a pivotable float which controls movement of a wiper contact over a resistor. Electrical connections to the wiper contact and the resistor result in a variable measure of resistance as the float moves in accordance with the level of fuel in the tank. A signal resulting from the variable resistance is sent to an indicator, such as a fuel gauge, to indicate to a vehicle operator the amount of fuel present in the tank.
The type of fuel sender assembly described above has achieved widespread use due to its simplicity and ease of manufacture relative to other designs, such as hydrostatic measuring systems or float systems with a mechanical linkage to a deflection gauge. Nevertheless, prior art pivotable float/wiper contact fuel senders are subject to several disadvantages.
In particular, prior art sender assemblies have required electrical calibration with the float at its zero level indicating position such that an electrical fuel gauge indicates properly at that level. For example, U.S. Pat. No. 4,532,491 issued to Rau et al, discloses a liquid level sender which has a swingably supported lever with a float on one end and a wiper contact moving along an electrical resistor on the other end. The fuel sender must be calibrated either by displacing a pair of stops relative to the wiper contact, by rotating the entire structure with respect to the fuel tank, or by making the electrical resistor displaceable with respect to the wiper contact within the structural part of the sender assembly.
In U.S. Pat. No. 3,842,673 issued to Riddel, a fuel level sending unit is described having a stationary arm and a movable arm, one of the arms including a wire-wound resistor. The movable arm is supported by a coil spring. Calibration of the fuel sender assembly is accomplished by reconfiguring one of the contact arms.
In addition to requiring calibration, prior art sender assemblies have been generally difficult to manufacture due to large part count and complicated mechanical linkages.
An improved structure employed in the prior art is characterized by a bent float rod which is rigidly attached to a carrier for rotation. The carrier carries a spring contact which slides over a resistor card. Since the spring contact and the float rod are in fixed relationship and since the resistor card location can be accurately controlled within the housing, a predictable resistance can be obtained from the unit at each rotational position of the float rod.
In this prior art assembly, stop limits for the rotation of the float rod have been provided by external posts on a housing for abutting the portion of the float rod extending toward the float. While calibration can be provided by choosing the relative locations of the post, the resistor card, and the carrier (i.e., calibration of individual units is not needed), the structure is subject to inaccuracy or failure caused by drift as the posts and float rod deform or by breakage of the posts. Inaccuracy is also caused in the prior art device by variation in the spring contact to resistor card contact point caused by variation in compression of the spring contact.
In copending application Ser. No. 289,883, a fuel sender assembly is described wherein a carrier element retains a pivot portion and an arm portion of a float rod. The carrier element subsection which receives the pivot portion of the rod is retained within a housing. The carrier element extends through a slot in the housing to a point where it receives the arm portion of the float rod. Also retained within the housing is a resistor card. A spring contact carried by the carrier element slidably engages the resistor card to derive the variable resistance.
A potential problem experienced with slidably engageable contacts is fretting. Vibrational forces cause the contact to repeatedly rub the same few resistor card conductor pads until the pads are worn away, thus causing an open circuit and a false measurement at that location. For example, automobiles are shipped by rail or truck from an assembly plant with some amount of fuel in their tanks. A single level of vibrating fuel often results in a particular area of fretting on the fuel sender resistor card due to vibrations transmitted by the float rod.